Heart failure (HF) is an increasingly prevalent disease that occurs when the cardiac muscle is unable to maintain a sufficient cardiac output. HF patients suffer, and eventually die from either progressive failure of cardiac mechanical function (pump failure) or ventricular arrhythmias. Pathological structural remodeling of the heart associated with myocyte hypertrophy and/or death is a common feature of HF. Altered Ca release from the sarcoplasmic reticulum (SR) due to deregulated cardiac ryanodine receptor (RyR2) function (i.e. leaky RyR2s) has been implicated in both contractile dysfunction and arrhythmias in HF as well as in the structural remodeling of the failing heart. However, direct evidence to causally link abnormal RyR2 function to HF is lacking. It also remains unclear how RyR2 abnormalities can lead to such differing manifestations of HF as contractile dysfunction and arrhythmogenesis. Similarly, the specific role and mechanisms of RyR2-mediated signaling in activation of hypertrophic pathways and cell death in the failing heart remains to be elucidated. This information is essential for understanding the pathophysiology of HF and for the development of novel, mechanistically-targeted HF therapies. Thus, we will directly examine the cause- effect relationships between abnormal RyR2-mediated Ca signaling and cardiac pathological processes using novel genetic models of dysregulated SR Ca release. Furthermore, the applicability of our findings to real HF will be tested in a clinically relevant model with confirmed RyR2 dysfunction. The specific aims are: 1) To determine the relationship between abnormal SR Ca release and contractile dysfunction in genetic models of Ca-dependent cardiac disease; 2) To define the specific intracellular signaling mechanisms that link dysregulated SR Ca release to pathological remodeling and myocyte death in genetic models of Ca-dependent cardiac disease; and 3):To test the hypothesis that dysregulated SR Ca release contributes to HF and probe normalization of Ca release refractoriness as a treatment strategy in a clinically relevant model of HF. We propose a systematic, integrated approach to test these hypotheses using advanced and state-of-the-art methods including single RyR2 channel recordings, cellular multi- compartmental Ca imaging, multicellular Ca imaging and contraction measurements, Ca- and membrane potential mapping in whole hearts, and measurements of in vivo electrical and mechanical function. Our systematic approach will allow us to directly address for the first time the specific mechanisms that causally link abnormal SR Ca release to contractile dysfunction and pathological remodeling and to explore new therapeutic strategies for HF by targeting these mechanisms.